The location of the pg1 (phosphogluconolactonase) gene of Escherichia coli on a 5.8 kb Kpn I fragment has been determined by insertional mutagenesis. The gene encodes a 42 KD protein, as observed in maxicell preparations. sapA, a gene involved in transpositional activation of the cryptic bg1 operon, appears to be identical to pg1 since mutations that inactivate one function inactivate the other and results in the loss of expression of the 42 KD protein.